1. Technical Field
This invention relates to multilayer metal gaskets for use in internal combustion engines and the like.
2. Related Prior Art
Multilayer metal gaskets find many uses in internal combustion engine applications as a means of sealing a fluid passage communicating between two components or members which are bolted together. Examples include gaskets used to seal the numerous passages between a head and block of an engine and those used to seal the passages between an exhaust manifold and the block through which hot gases pass from the combustion chambers into the manifold.
Some applications call for a metal gasket of a three-layer configuration, including a pair of outer active layers which are fabricated of a resilient metal material, such as high alloy steels, separated by an intermediate layer. Typically, the outer active layers are formed with at least one set of lined apertures defining an opening in the gasket surrounded by at least one sealing bead formed as an annular deformation in each of the outer active layers. The sealing bead presents outer sealing surfaces which contact and seal against the mating surfaces of the two bolted members. As the two members are drawn together during assembly, the sealing beads are compressed elastically under load, providing fluid-tight sealing engagement between the sealing surfaces of the outer layers and the mating surfaces of the two members being sealed. The intermediate metal layer is formed with an aperture aligned with the apertures of the outer active layers, having a thickened annular stopper portion or ring surrounding such aperture and presenting upper and lower stopper surfaces surrounding the opening which are in contact with or are engageable with the inner surfaces of the active layers. The thickness of the stopper layer together with the material thickness of the active layers in contact with the stopper layer define a theoretical maximum closing of the gap between the members, and hence a maximum limit of deformation of the resilient sealing beads. The function of the stack of material layers in the stopper region is to prevent over compression of the sealing beads beyond the elastic limit which would cause irrecoverable plastic deformation of the sealing beads and hence damage to the gasket.
If the manufacture and installation of the gasket is not carefully controlled, variations in material thicknesses and/or excessive loading on installation and use can cause over compression of the sealing bead beyond its elastic limit. Particularly in exhaust manifold gasket applications, the sealing surfaces of the outer active layers are exposed to the extremely hot exhaust gases passing through the sealed opening from the block to the exhaust manifold. Prolonged exposure of the outer active layers to such hot exhaust gases can deteriorate the elastic properties of the outer active layers by effectively softening the material and impairing their sealing capabilities.
Gaskets used in such environments are also subjected to extreme thermal cycling and high thermal loads as the gaskets are exposed to repeated hot and cold engine conditions. Under hot engine conditions, the expansion of the bolted members can impart additional compressive loads on the gasket, exceeding the compressive yield strength of the stopper portion of the gasket, that can lead to a condition know as “thermal crush” in which the sealing beads of the outer active layers are further compressed beyond their designed elastic limit, leading to permanent, irreversible plastic deformation of the sealing beads, impairing the sealing capabilities of the gasket.
A gasket constructed according to the invention overcomes or greatly minimizes the foregoing limitations associated with such prior multilayer metal gaskets.